8.1+Forming+Chemical+Bonds

The force that holds two atoms together is called a chemical bond. Chemical bonds form because of attractions between oppositely charged atoms, called ions, or between electrons and nuclei. The outermost, or valence, electrons of atoms are the ones mainly involved in the formation of bonds. The elements within a group of the periodic table typically have the same number of valence electrons.

Elements tend to react so as to achieve the stable electron configuration of a noble gas, typically an octet of electrons. A cation, or positive ion, is formed when an atom loses one or more electrons. An anion, or negative ion, is formed when an atom gains one or more electrons. The periodic table is useful in predicting the charges of ions typically formed by various atoms.

Example: Calcium (Ca, atomic number 20) is an element in group 2A of the periodic table. Write the electron configuration for a neutral atom of calcium. Tell how many electrons this atom readily tends to gain or lose to form an ion. Predict the charge on the ion, write its formula, and tell whether it is a cation or an ion. Finally, write the electron configuration of this element.

A neutral atom of element 20 would have 20 electrons, giving it the electron configuration 1s2.2s2.2p6.3s2.3p6.4s2. To achieve the stable electron configuration of a noble gas, the atom would tend to lose its two valence electrons, producing a cation with a charge of 2+ and the formula Ca2+. The configuration of this ion would be 1s2.2s2.2p6.3p6.